"We need to find new villains, we need to find new ideas, otherwise the show doesn't keep growing," said Moffat.

"Moriarty's a one-shot deal in the original [books]. I don't want [Sherlock] to turn into a show that's about one villain and one hero. Moriarty was great because he was a surprise. Every time you bring him back he won't be as big a surprise."

"We have to keep the show growing otherwise it won't surprise you," he suggested. "Once you get the measure of it you'll just get critical - if we keep surprising you and keeping you off balance, you won't."

No air date has been announced for the third series of Sherlock, though executive producer Sue Vertue has claimed that the detective drama is unlikely to return before 2013.